One in flight of birds finally falling to rook (7)
I believe the answer is:
aviator
'one in flight' is the definition.
(an aviator flies)
'of birds finally falling to rook' is the wordplay.
'of birds' becomes 'avian' (avian means relating to birds).
'finally falling' means to remove the last letter.
'rook' becomes 'R' (chess abbreviation).
'avian' with its final letter taken away is 'avia'.
'avia'+'to'+'r'='AVIATOR'
(Other definitions for aviator that I've seen before include "Flier" , "Woman in a flying machine" , "Navigator of the air" , "Pilot; style of sunglasses" , "One flying aircraft" .)